Straw is the collection and baling of the stems of a grain crop left over after the grain has been harvested. Wheat is the most common straw used for bedding, but oats, rye and barley are also used.
I started my career with horses on a farm that used wheat straw bedding. The most significant advantage of this material is the “Pampers effect,” where urine flows through the straw and into the ground below. This keeps the horse dry. Sometimes the urine would pool on the solid dirt, but it never remained next to the horse.
Avoid using rye straw with pregnant mares. This may be anecdotal, but many believe that the fungus that grows on the live rye plant that causes abortion (ergot) remains on the dried rye straw. Rather than challenge this, I just recommend not using rye straw.
All straw bedding can mold and should be avoided when found. Another issue is that some horses like to eat straw, especially oat straw. Straw remains popular on the Thoroughbred race tracks but is rarely seen on other farms. Most new horse owners have never seen straw and wouldn’t know how to clean the stall bedded with it. It requires a pitchfork with metal tines.
Recommended particular uses for straw are post-operation and foal delivery. Surgical wounds often collect bits of debris which can also enter open wounds such as an open castration site, causing infection. Shavings and sawdust can get into a foal’s nostrils and clog them where straw cannot.